HIV-1, the causative agent of AIDS, spreads to T cells much more efficiently via cell-to-cell transmission than via cell-free infection. In tissue-culture-based studies, HIV-1-infected T cells and uninfected target T cells form cell contact structures known as the virological synapses (VS) at which massive virus transfer is likely to take place. However, the nature of cell-to-cell transmission in the context of lymphoid tissues where the contact-mediated virus spread is likely favored remains poorly understood. Cell-to-cell virus transmission is not only efficient in spreading HIV-1 but also shown to help HIV-1 to evade humoral immune response and effects of a few antivirals. Furthermore, in lymphoid organ environment, cell-to-cell transmission from HIV-1-infected T cells to non-permissive resting T cells causes abortive infection, which leads to massive death of the resting T cells. In addition, infection of resting T cells in the presence of lymphoid tissue chemokines produced by stromal cells leads to viral latency. Therefore, fully elucidating the mechanisms promoting cell-to-cell transmission, especially in lymphoid tissue environment, is critical for our understanding of HIV-1 pathogenesis. T cell zones of lymphoid tissues contain stromal cells in addition to T cells and antigen-presenting cells. These stromal cells form a dense network and play structural and regulatory roles for T cell interactions and trafficking. Although the effects of the T cell zone stromal cells on T cell behaviors have become clear, the role played by these stromal cells in HIV-1 spread is undetermined, partly due to the lack of tractable model systems. Notably, our preliminary data suggest that the presence of stromal cells in a coculture system alters the efficiency of cell-to-cell HIV-1 spread. Based on these observations, we hypothesize that lymphoid tissue stromal cells facilitates cell-to-cell HIV-1 spread and thereby promotes viral propagation, latency or cell death. To address this hypothesis, in this exploratory grant application, we plan [1] to determine the effects of stromal cells on VS formation and cell-to-cell HIV-1 transmission and [2] to elucidate roles played by stromal cells in infection of resting T cells and subsequent processes leading to latency and/or cell death. In these studies, the properties of cell contacts, the nature of virus spread, and the fates of infected cells will be examined using newly developed coculture systems, which model the T cell zone environment. These studies are designed to fill the knowledge gaps in our understanding of HIV-1 spread in lymphoid organs. They are also expected to provide the field with a model system in which virus spread and fates of infected cells can be analyzed in a physiologically relevant yet tractable environment.